Security systems are presently limited in their ability to detect contraband, weapons, explosives, and other dangerous objects concealed under clothing. Metal detectors and chemical sniffers are commonly used for the detection of large metal objects and certain types of explosives; however, a wide range of dangerous objects exist that cannot be detected using these devices. Plastic and ceramic weapons increase the types of non-metallic objects that security personnel are required to detect. Manual searching of subjects is slow, is inconvenient, and would not be well tolerated by the general public, especially as a standard procedure in high traffic centers, such as airports.
It is known in the art that images of various types of material can be generated using X-ray scattering. The intensity of scattered X-rays is related to the atomic number (Z) of the material scattering the X-rays. In general, for atomic numbers less than 25, the intensity of X-ray backscatter, or X-ray reflectance, decreases with increasing atomic number. Images are primarily modulated by variations in the atomic number of the subject's body. Low-Z materials present a special problem in personnel inspection because of the difficulty in distinguishing the low-Z object from the background of the subject's body which also has low-Z.
Known prior art X-ray systems for detecting objects concealed on persons have limitations in their design and method that prohibit them from achieving low radiation doses, which is a health requirement, or prevent the generation of high image quality, which are prerequisites for commercial acceptance. An inspection system that operates at a low level of radiation exposure is limited in its precision by the small amount of radiation that can be directed against a person being searched. X-ray absorption and scattering further reduces the amount of X-rays available to form an image of the person and any concealed objects. In prior art systems this low number of detected X-rays has resulted in unacceptably poor image quality.
This problem is even more significant if an X-ray inspection system is being used in open venues such as stadiums, shopping malls, open-air exhibitions and fairs, etc. This is because that in such venues, people can be located both proximate to and/or at a distance from the machine. If a person being scanned is not very close to the X-ray machine, the obtained image may not be clear enough since the amount of radiation reaching the person is very low. This limits the range of scanning of the system to a few feet from the front of the machine. If, however, a person being scanned is too close to the X-ray machine, the amount of radiation impinging on the person may not be safe.
The amount of radiation exposure caused by known X-ray screening systems is commonly limited by the beam chopping apparatus employed in the systems.
Conventional beam chopping mechanisms generally consist of a disc wheel with collimator slits embedded therein at fixed distances from one another. The disc wheel is spun at a particular velocity and an X-ray beam of a particular energy is directed into more focused beams when passing through slits of the chopper wheel. The conventional chopper wheel is described in greater detail below throughout the specification in reference to the present disclosure.
It should be understood by persons having ordinary skill in the art that radiation sources are typically very heavy. In order to accommodate for the weight of the X-ray source, a chopper wheel configuration, as employed in the prior art, would need to be rather large. This substantially increases the weight of the system and makes it less portable. In addition, the chopper wheel, as employed in the prior art, is fraught with balance and gyroscopic effects. For example, the gyroscopic effect can be likened to a gyroscope toy where a string is pulled (such as a spinning top). As the top is spun fast, fluctuations in motion are not discernable, but, once it slows down, the top will start to wobble and vibrate. Thus, there is a certain RPM that must be kept to maintain balance. In addition, with increasing weight there are issues of humming noises at higher RPMs. In order to overcome the challenges in using conventional chopper wheel configurations, mechanical manipulation of the speed and size of the chopper wheel is necessary.
Therefore, what is needed is a beam chopping apparatus, and more specifically, a helical shutter for an electron beam system, that allows for variability in both velocity and beam spot size by modifying the physical characteristics or geometry of the beam chopper apparatus.
There is also need for a beam chopping apparatus, and more specifically, a helical shutter for an electron beam system, which provides a vertically moving beam spot with constant size and velocity to allow for equal illumination of the target.
Further, there is need for a beam chopping apparatus, and more specifically, a helical shutter for an electron beam system, which creates a wider field of view during operation.
Also, there is need for a beam chopping apparatus, and more specifically, a helical shutter for an electron beam system, that is lightweight and does not cause an X-ray source assembly employing the beam chopper to become heavy and difficult to deploy.